Use WHITE and BLACK constants where appropriate

rtchallenge/src/canvas.rs

@@ -74,17 +74,16 @@ impl Canvas {
 mod tests {
     use super::Canvas;
 
-    use crate::tuples::Color;
+    use crate::{tuples::Color, BLACK};
 
     #[test]
     fn create_canvas() {
-        let bg = Color::new(0.0, 0.0, 0.0);
+        let bg = BLACK;
         let c = Canvas::new(10, 20, bg);
         assert_eq!(c.width, 10);
         assert_eq!(c.height, 20);
-        let black = Color::new(0., 0., 0.);
         for (i, p) in c.pixels.iter().enumerate() {
-            assert_eq!(p, &black, "pixel {} not {:?}: {:?}", i, &black, p);
+            assert_eq!(p, &BLACK, "pixel {} not {:?}: {:?}", i, &BLACK, p);
         }
     }